1. Field of the Invention
This invention relates generally to the field of petroleum coking processes, and more specifically to modifications of petroleum coking processes for the production of a premium-quality, “fuel-grade” petroleum coke. This invention also relates generally to the use of this new formulation of petroleum coke for the production of energy, and more specifically to modifications in conventional, solid-fuel furnaces and environmental control systems to take optimal advantage of its unique properties.
2. Description of Prior Art
Since initial efforts to refine crude oil in the U.S. during the late 1800s, the search for an appropriate use for the heaviest fractions of crude oil (i.e. the “bottom of the barrel”) has been a perplexing problem. Initially, many refineries received little to no value from the heaviest fractions of crude oil. Some were noted to simply discard the “bottom of the barrel.” Over time, some of the heavy crude oil fractions were used in asphalt products and residual fuel oils. However, the demand for these products was not sufficient to consume increasing production.
As demand for transportation fuels (e.g. gasoline, diesel, and aviation fuels) increased in the early 1900s, thermal cracking processes were developed to convert the heavy crude oil fractions into lighter products. These refinery processes evolved into the modern coking processes that predominate the technology currently used to upgrade the heaviest fractions of the crude oil. These processes typically reduce the quantity of heavy oil fractions, but still produce unwanted by-products (e.g. petroleum coke) with marginal value.
A. Production of Petroleum Coke: Coking Processes
In general, modern coking processes employ high-severity, thermal decomposition (or “cracking”) to maximize the conversion of very heavy, low-value residuum feeds to lower boiling hydrocarbon products. Coker feedstocks typically consist of non-volatile, asphaltic and aromatic materials with “theoretical” boiling points exceeding 1000° F. at atmospheric pressure. The boiling points are “theoretical” because these materials coke or crack from thermal decomposition before they reach such temperatures.
Coking feedstocks normally consist of refinery process streams which cannot economically be further distilled, catalytically cracked, or otherwise processed to make fuel-grade blend streams. Typically, these materials are not suitable for catalytic operations because of catalyst fouling and/or deactivation by ash and metals. Common coking feedstocks include atmospheric distillation residuum, vacuum distillation residuum, catalytic cracker residual oils, hydrocracker residual oils, and residual oils from other refinery units. Consequently, coking feedstocks vary substantially among refineries. Their composition and quantity primarily depend on (1) the input crude oil blend, (2) refinery processing equipment, and (3) the optimized operation plan for any particular refinery. In addition, contaminant compounds, which occur naturally in the crude oil, generally have relatively high boiling points and relatively complex molecular structures. Consequently, these contaminant compounds, containing sulfur and heavy metals, tend to concentrate in these residua. Many of the worst process streams in the refinery have become coker feedstock, and their contaminants usually end up in the petroleum coke by-product. For this reason, the coking processes have often been labeled as the “garbage can” of the refinery.
There are three major types of modern coking processes currently used in refineries to convert the heavy crude oil fractions into lighter hydrocarbons and petroleum coke: Delayed Coking, Fluid Coking™, and Flexicoking™. In all three of these coking processes, the petroleum coke is considered a by-product that is tolerated in the interest of more complete conversion of refinery residues to lighter hydrocarbon compounds, referred to as “cracked liquids” throughout this discussion. These cracked liquids range from pentanes to complex hydrocarbons with boiling ranges typically between 350 and 950° F. The heavier cracked liquids (e.g. gas oils) are commonly used as feedstocks for further refinery processing that transforms them into transportation fuel blend stocks.
The delayed coking process has evolved with many improvements since the mid-1930s. Essentially, delayed coking is a semi-continuous process in which the heavy feedstock is heated to a high temperature (between 900° F. and 1000° F.) and transferred to large coking drums. Sufficient residence time is provided in the coking drums to allow the thermal cracking and coking reactions to proceed to completion. The heavy residua feed is thermally cracked in the drum to produce lighter hydrocarbons and solid, petroleum coke. One of the initial patents for this technology (U.S. Pat. No. 1,831,719) discloses “The hot vapor mixture from the vapor phase cracking operation is, with advantage, introduced into the coking receptacle before its temperature falls below 950° F., or better 1050° F., and usually it is, with advantage, introduced into the coking receptacle at the maximum possible temperature.” The “maximum possible temperature” in the coke drum favors the cracking of the heavy residua, but is limited by the initiation of coking in the heater and downstream feed lines, as well as excessive cracking of hydrocarbon vapors to gases (butane and lighter). When other operational variables are held constant, the “maximum possible temperature” normally minimizes the volatile material remaining in the petroleum coke by-product. In delayed coking, the lower limit of volatile material in the petroleum coke is usually determined by the coke hardness. That is, petroleum coke with <8 wt. % volatile materials is normally so hard that the drilling time in the decoking cycle is extended beyond reason. Various petroleum coke uses have specifications that require the volatile content of the petroleum coke by-product to be <12%. Consequently, the volatile material in the petroleum coke by-product typically has a target range of 8-12 wt. %. Prior art in the delayed coking process, including recent developments, has attempted to maximize the production of cracked liquids with less coke production. In this manner, the prior art of delayed coking has attempted to minimize coke yield and the amount of volatile materials it contains.
Fluid Coking™, developed since the late 1950s, is a continuous coking process that uses fluidized solids to increase the conversion of coking feedstocks to cracked liquids, and further reduce the volatile content of the product coke. In Fluid Coking™, the coking feedstock blend is sprayed into a fluidized bed of hot, fine coke particles in the reactor. Since the heat for the endothermic cracking reactions is supplied locally by these hot particles, this permits the cracking and coking reactions to be conducted at higher temperatures (about 480-565° C. or 900-1050° F.) and shorter contact times than in delayed coking. Roughly 15-25% of the coke is burned in an adjacent burner vessel in order to create the hot coke nuclei to contact the feed in the reactor vessel, and satisfy the process heat requirements. The Fluid Coking™ technology effectively removes the lower limit of volatile content in the petroleum coke, associated with delayed coking. The volatile content of the petroleum coke produced by the Fluid Coking™ technology is typically minimized (or reduced), within the range of 4-10 wt. %. Consequently, the quantity of petroleum coke, produced by a given feedstock, and its volatile content are significantly reduced in the Fluid Coking™ technology (vs. delayed coking).
Flexicoking™ is an improvement of the Fluid Coking™ process, in which a third major vessel is added to gasify the product coke. A coking reactor, a heater (vs. burner) vessel, and a gasifier are integrated into a common fluidized-solids circulating system. The “cold coke” from the reactor is partially devolatilized in the heater vessel. In the gasifier, over 95% of the gross product coke is gasified to produce either low heating-value fuel gas or synthesis gas to make liquid fuels or chemicals. In this manner, the net coke yield is substantially reduced. The purge coke (˜5% of the product coke) from the Flexicoking™ process normally contains about 99% of the feed metals and has a volatile content of 2-7 wt. %.
Through the years, improvements in the coking processes have been substantially devoted to increasing the yield and recovery of cracked liquids and decreasing the coke yield. Thus, the content of volatile material in the resulting petroleum coke has been continually decreased, where possible. Various patents disclose improvements to the delayed coking process that include, but are not limited to, (1) coker designs that reduce drum pressures (e.g. 25 to 15 psig), (2) coker designs to provide virtually no recycle, and (3) periodic onstream spalling of heaters to increase firing capabilities and run length at higher heater outlet temperatures. These technology advances have been implemented in an effort to maximize the cracked liquid yields of the delayed coker and reduce petroleum coke yields and volatile content.
Other modifications of these coking processes introduce various wastes for disposal. Several patents disclose various means to inject certain types of oily sludges. Other prior art uses these coking processes for the disposal of used lubricating oils. Additional patents disclose the use of these coking processes for the disposal of other wastes. In general, these patents discuss the potential limited impact on the coke yield and volatile content, and promote other means to negate any increases. Also, these waste disposal techniques often increase the ash content of the coke and can introduce additional, undesirable impurities, such as sodium. Consequently, the objectives of these patents are to reuse or dispose of these wastes rather than enhance the petroleum coke properties.
B. Uses of Petroleum Coke
The uses of the petroleum coke by-products from these coking processes depend primarily on its (1) physical properties and (2) chemical composition (i.e. degree of contamination). The physical properties (density, crystalline structure, etc.) of the petroleum coke by-product are determined by various factors, including coking feedstock blend, coking process and operation, and volatile content of the coke. The chemical composition and degree of contamination of the petroleum coke is primarily determined by the composition of the coking feedstock blend. That is, most of the contaminant compounds (e.g. sulfur, nitrogen, and various metals) in the petroleum coke by-product come from heavy, complex chemical structures in the coking feedstocks, which normally come from the refinery's crude oil blend. Conversely, the contaminants in the refinery's crude oil blend ultimately concentrate in the petroleum coke. Consequently, light, sweet crudes generally have less contaminants and allow the production of higher value petroleum coke by-products. However, crude oils are becoming increasingly heavy and sour, increasing the production of low-grade petroleum coke.
Premium and intermediate grades of petroleum cokes have low to moderate levels of sulfur (e.g. 0.5-2.5%) and heavy metals (vanadium, nickel, etc.). These grades of coke have various uses as electrodes and metallurgical carbon in the production of aluminum and steel. In some applications, the raw petroleum coke is further processed by calcining to remove volatile material and increase the coke density. Petroleum coke that cannot meet the required specifications of these higher-value markets is classified as “fuel-grade” petroleum coke. As such, this poorest grade of petroleum coke typically has high concentrations of sulfur (2.5-5+ wt. %) and/or heavy metals, including vanadium and nickel.
“Fuel-grade” petroleum coke is actually a misnomer. The traditional “fuel-grade” petroleum coke actually performs very poorly as a fuel. First of all, traditional “fuel-grade” petroleum coke cannot sustain self-combustion due to its poor fuel properties and combustion characteristics. Secondly, its high sulfur content (e.g., >2.5 wt. %) creates substantial environmental problems, particularly in the United States. Thirdly, high concentrations of certain metals can be precursors for post-combustion, liquid salts that deposit on heat transfer surfaces, reducing efficiency and/or causing accelerated corrosion. Finally, high concentrations of sulfur and/or metals can detrimentally effect product quality, when used as fuel directly in chemical processes (e.g. concrete kilns). Consequently, traditional “fuel-grade” petroleum coke can only be used in conventional furnaces when combined with other fuels (often requiring separate fuel processing and management systems). Alternatively, specially designed combustion systems, that are cumbersome and expensive, can use this coke as fuel. Until these deficiencies are addressed, the traditional “fuel-grade” petroleum coke will continue to be a very low value product. In fact, traditional “fuel-grade” petroleum coke could be classified as a hazardous waste in the United States, if its value continues its downward trend and refiners receive no sales value as a product. In this scenario, costs of hazardous waste disposal could dramatically reduce refinery profitability, and cause the shutdown of many refineries across the United States.
Numerous technologies were apparently developed to modify coking feedstocks and produce petroleum coke of sufficient quality for non-fuel uses of higher value. Many patents disclose various technologies for removing or diluting certain undesirable contaminants in the petroleum coke. As such, they go far beyond the degree of decontamination that is required for petroleum coke used as a fuel. Accordingly, simpler approaches that are less expensive and less complicated are desirable for the lower level of decontamination required for petroleum coke used as a fuel.
Various combustion technologies have been developed to overcome the deficiencies in “fuel-grade” coke, but no prior art successfully addresses these problems by upgrading the coke via the coking process. The prior art has failed to upgrade the quality of “fuel-grade” petroleum coke sufficiently to use in conventional, solid-fuel combustion systems (e.g. high heat capacity furnaces with suspension burners firing pulverized fuel, such as coal). Specially designed combustion systems (noted above) include fluidized bed combustion, pyrolysis/gasification systems, and low heat capacity furnaces (i.e. without heat absorption surfaces). In general, these systems are cumbersome, expensive, and have significant problems in scaling size upward. Several patents also disclose technologies to grind and stabilize coke/oil mixtures for use in conventional combustion systems. However, the quality of the traditional petroleum coke used in these fuel mixtures normally limits (1) the particle size distribution of the solids and (2) the degree of combustion (i.e. carbon burnout).
In summary, prior art does not address the major problems associated with traditional “fuel-grade” petroleum coke:
1. There remains a major need to produce “fuel-grade” petroleum coke that is able to sustain self-combustion with acceptable combustion efficiencies.
2. Secondly, no known prior art satisfactorily resolves the problems associated with the formation of sticky, corrosive salts in the combustion process, due to certain contaminants in the petroleum coke.
3. Finally, prior art does exist for the desulfurization and demetallization of petroleum coke, but it is complicated and expensive. Simpler approaches are needed for the lower level of decontamination required for petroleum coke used as a fuel.